This invention relates to compressive load carrying bearings and more particularly to laminated bearings of the type comprising alternating bonded layers of a resilient material such as an elastomer and a non-extensible material such as a metal.
It is well known that the compressive load carrying ability of a given thickness of an elastomer material may be increased many times by subdividing it into a plurality of layers and separating the layers by intervening layers of a non-extensible material. At the same time, however, the ability of the resilient material to yield in shear or torsion in a direction parallel to the layers is substantially unaffected. This concept has been adopted or utilized in the design of different forms of laminated bearings, as exemplified by the following U.S. Patents: Dolling, No. 3,941,433; Lee et al, No. 3,429,622; Schmidt, No. 3,679,197; Boggs, No. 3,377,110; Krotz, No. 3,179,400; Orain, No. 2,995,907; Hinks, No. 2,900,182; and Wildhaber, No. 2,752,766, and the prior art cited therein. Laminated elastomeric bearings of various types are commonly used in commercial applications where it is necessary to carry large compressive loads in a first direction and also to accommodate limited relative movement in other directions. The bearings are designed so that the large compressive loads are carried generally perpendicular to the resilient lamellae. A significant commercial variety of bearings is characterized by the alternating bonded lamellae being disposed concentrically about a common center, i.e., so that successive alternating layers of resilient and nonextensible materials are disposed at successively greater radial distances from the common center. This variety of bearings includes a number of different configurations, notably bearings which are cylindrical, conical or generally spherical in shape or which are essentially sectors of cylinders, cones and spheres.
Conically-shaped laminated bearings may be used for a variety of applications, but have achieved particular importance in helicopters where they are employed as main bearings for the blade shafts of the main rotor. In the typical helicopter application, the conically-shaped bearing is required to accommodate cyclic torsional motion about a given axis while simultaneously carrying a large compressive load along that axis, and radial loads along an axis perpendicular to the bearing center line. The result is that greater compressive stresses and shear stresses and strains are established in the resilient layers closest to the common center and failure from fatigue encuntered in accommodating the torsional motion tends to occur at the innermost resilient layer.
For the usual laminated bearing application it is desirable, if not essential, to have a bearing design which provides an optimum combination of load-carrying capability, spring rate and strain distribution consistent with cost and life expectancy considerations. A bearing of conical geometry employed in a helicopter main rotor retention system is required to undergo dynamic and static torsional deflection as well as dynamic and static compressive loading. The bearing experiences shear strain produced from torsional deflections about the bearing center line. This torsional sheat strain is not uniformly distributed and will vary in distribution as a function of the magnitude of torsional deflection. Additionally shear strains are induced by application of compressive loads (either axial or radial) and these shear strains are maximum at the edges of the elastomer layers located along the apex (the inner circumference) of the bearing. The edges of the elastomer layers tend to bulge from between the adjacent non-extensible laminations under compressive loading, thereby exposing more of those layers to wear. The wear on the elastomeric layers tends to be much greater at their inner (apex) edges than elsewhere due to the higher strain levels in the apex region, with the result that the bearing will usually fail due to extrusion and fatigue of the elastomer layers at their inner edges. Of course the tendency to bulge can be reduced by making the elastomer layers of a material having a greater modulus of elasticity. However, increasing the modulus will increase the torsional, radial and axial spring rates of the bearing. In the typical helicopter application, increasing the spring rate may not be acceptable since it may result in the need for a concomitant increase in the power capability (and/or a decrease in the useful life) of the actuator or other device which is coupled to the bearing. Furthermore, the bulging problem at the outer circumference of the bearing may not be sufficiently severe due to the lower level of compressive load-induced strain as to require any increase in modulus. On the other hand an increase in modulus may increase the torsional spring rate at the outer circumference beyond acceptable limits. In this connection it should be noted that, on the basis of computer finite element analysis of the elastomer layers, increasing the modulus of an elastomer layer will produce a greater contribution to the torsional spring rate of an element of the layer located at its base end than an element of the same length located at its apex end, due to the difference between the effective radii of such elements. Hence merely changing the modulus of each layer to reduce the compression-induced strains at the apex side of the bearing usually is not a practical solution since it makes it difficult to achieve an optimum combination of compression-induced edge shear strain, torsional shear strain distribution and lowest possible torsional spring rate consistant with the cost, lifetime and operating requirements of the system in which the bearing is mounted.
It has been recognized that absolute uniformity of compression induced shear strains within a layer is impossible to achieve because the strains decrease from some finite value at each of its exposed edges to zero at some point intermediate those edges. Nevertheless the more uniform the compression induced shear strains become within each elastomer layer between its edges, the less likely that one layer will fail a substantial time before the other layers. The same is true if the strains in adjacent layers are made more nearly the same at corresponding points. In this connection it is to be noted that because of differences between the average radius of the layers of a conical bearing, the compressionally-induced and torsionally-induced shear strains may tend to vary substantially on a layer-to-layer basis where all of the elastomer layers have the same modulus of elasticity and thickness.
Schmidt, supra, proposed to improve the fatigue life of bearings by progressively increasing the thicknesses of successive layers of resilient material with increasing radius and simultaneously to progressively decrease the modulus of elasticity of those same layers with increasing radius. However, the Schmidt technique is expensive in that it requires that each elastomer layer be made of a different material. Thus, an elastomeric bearing consisting of fifteen resilient layers necessitates provision of fifteen different elastomer materials. Even though this may be achieved by subdividing a basic elastomer feedstock into fifteen lots and modifying each lot with a different amount or type of additive, the fact remains that it is costly, time consuming and inconvenient to provide a different material for each resilient layer. Furthermore, care must be taken to assure that the materials are properly identified so that they will be correctly arranged with modulus of elasticity decreasing with increasing radius as prescribed by Schmidt.